The present invention relates to a color reproduction system capable of reproducing a color of an object in consideration of observation illumination light when the image recorded on a recording medium is to be reproduced.
Recently, a color management system (CMS) such as a color reproduction system has been widely used, in which the image data obtained by photographing a desired object using a color image input device such as a digital camera is corrected, and a color of the object is faithfully reproduced by a display device such as a CRT monitor or an output device such as a color printer.
In such a system, if illumination light on the photographing side on which an object is photographed differs from illumination light on the observation side on which a reproduced image is observed, the color based on the tristimulus values X, Y, and Z of the object on the photographing side and observed under the illumination light on the observation side looks different from that under the illumination light on the photographing side owing to changes in the perception characteristics of a person, e.g., color adaptation. That is, a problem is posed in terms of “appearance”.
The tristimulus values X, Y, and Z are the quantitative values of a color which are determined by if the International Commission on Illumination (Commission Internationale de l'Eclariage; CIE) and guarantee the “appearance” of the same color under the same illumination light. However, such values cannot properly cope with the “appearance” of a color under different kinds of illumination light as described above.
In order to solve this problem, a conventional CMS aims at reproducing corresponding colors as tristimulus values that provide the “appearance” of the same color on both the observation side and the photographing side by using a human chromatic perception model such as a color adaptation model.
Several models as human color perception models including chromatic adaptation models are disclosed in Mark. D. Fairchild, Color Appearance Models, Addison Wesley, (1998). Studies have been made to construct models that allow more accurate color prediction.
The conventional CMS reproduces the “appearance” of a color of an object on the photographing side. In contrast to this, the color reproduction system disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-172649 estimates a spectral reflectance image of an object from a photographing image, and applying an observation illumination light spectrum to the spectral reflectance image to obtain tristimulus values under the observation illumination light and reproduce the color, thereby reproducing the “appearance” of the color on the observation side.
In a method of performing such illumination conversion, since tristimulus values of a color of an object under actual observation illumination light are reproduced, the “appearance” of the color can be accurately reproduced without any consideration given to the human perception characteristics such as chromatic adaptation.
Unlike in the conventional CMS, in the above color reproduction method of performing illumination conversion, there is no need to establish a linear conversion relationship between the spectral sensitivity of a camera and CIEXYZ color matching functions, and a color adaptation model under study need not be used. This method, however, requires various data, e.g., illumination light spectrum data on the photographing side and observation side, the spectral sensitivity data of a camera, and statistical characteristics of the spectral reflectance of an object.
In order to measure illumination light spectrum data of such data, an expensive spectrophotometer is generally required, and measurements under the actual photographing environment and observation environment are required. This makes it difficult to construct a simple system.
In addition, the spectrum characteristics of observation illumination light are not always suited to reproduce a color of an object, a problem arises when delicate differences between colors are to be evaluated.
When there are a plurality of portions to be observed, these portions are rarely positioned under the same illumination light. In general, the respective observers use different kinds of illumination light, and observe the color under the different kinds of illumination light. An inconvenience is caused in many cases, e.g., when an observer is to make a decision for diagnosis in a remote medical practice on the basis of colors while observing a color image of a patient and when absolute evaluation is required without any dependence on communication and observation environments.